Hundreds of volumes of records from the Scottish Catholic Archives became available on the ScotlandsPeople.gov.uk website.
First Minister Alex Salmond was joined by Cardinal Keith O’Brien, head of the Catholic Church in Scotland, at the official launch in Edinburgh.
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The archives include details of French royal family members who sought refuge in Scotland after the revolution and Sherlock Homes creator Sir Arthur Conan Doyle’s family tree.
Mr Salmond said: “This treasure trove of information will give Scots and homecomers an understanding of our shared history as well as an exciting opportunity to trace their family background.
“The interesting insights into the life of Sir Arthur Conan Doyle and the French royal refugees are only some of the stories to be discovered using this valuable resource.”
Conan Doyle, who never used his baptismal middle name, Ignatius, was born and brought up Catholic at Picardy Place in Edinburgh, though he later renounced religion.
The archives show that in the period after the revolution in France, the French royal family were refugees in Scotland and were put up in Holyrood Palace.
The records date from 1703 to 1908 and cover Catholic congregations across Scotland. Civil registrations only started in Scotland in 1855.
Ministers hope the archives will help attract some of Scotland’s 40 million diaspora to explore their heritage in the year of Homecoming.
Cardinal O’Brien said: “This is an extremely exciting project which will be of immense interest to professional and amateur genealogists for many years to come.
“Online access allows researchers anywhere in the world to glimpse the fascinating detail of the lives of past generations.”
The two-year project adds an extra 143,000 pages and two million new names to the 65 million already on the database.
George MacKenzie, Keeper of the Records of Scotland added: “We expect a lot of interest in this new service from people with Scots ancestry right across the world.”